docs: def get_volume(self):
          """Return the current volume level."""
          params = '<InstanceID>0</InstanceID><Channel>Master</Channel>'
          res = self.soap_request(URL_CONTROL_DMR, URN_RENDERING_CONTROL,
                                  'GetVolume', params)
          root = ET.fromstring(res)
          el_volume = root.find('.//CurrentVolume')
          return int(el_volume.text)